1. Field of the Invention
This invention relates to autodeposition. Autodeposition involves the use of an aqueous resinous coating composition of relatively low solids concentration (usually less than about 10%) to form a coating of relatively high solids concentration (usually greater than about 10%) on a metallic surface immersed therein, with the coating increasing in thickness and areal density (mass per unit area of coating) the longer the time the metallic surface is immersed in the composition. Autodeposition is somewhat similar to electrodeposition but does not require the aid of external electrical current to cause the resin particles to deposit on the metal surface.
In general, autodepositing compositions are aqueous acid solutions having solid resin particles dispersed therein in very finely divided form. The coating formed while the metal substrate used is immersed in the bath is generally wet and fairly weak, although sufficiently strong to maintain itself against gravity and moderate spraying forces. In this state the coating is described as "uncured". To make an autodeposition coated object suitable for normal practical use, the uncured coated is dried, usually with the aid of heat. The coating is then described as "cured".
The present invention relates more particularly to the use in autodeposition of blends of resins of at least two different chemical types to achieve resistance to corrosion, particularly in environments subject to mechanical shocks, for metallic objects coated with an autodeposited coating. In some of its embodiments, this invention also relates to chemical treatment of an uncured autodeposited coating for the purpose of improving various properties thereof, particularly the adhesion of the coating to the underlying metal substrate and the resistance to corrosion of the underlying metal provided by the cured autodeposited coating when the coated metal surfaced object is subjected to corrosive environments. The chemical treatments used for such a purpose are often called "reactive [or reaction] rinses" in the art. Further details about this aspect of the invention are given in copending U.S. application Ser. Nos. 07/718,676 filed Jun. 21, 1991 and 07/645,435 filed Jan. 24, 1991 by the same applicant. The entire disclosures of these two U.S. Applications, to the extent not inconsistent with any explicit statement herein, are hereby incorporated herein by reference.
2. Statement of Related Art
Basic constituents of an autodepositing composition are water, resin solids dispersed in the aqueous medium of the composition, and activator, that is, an ingredient or ingredients which convert the composition into one which will form on a metallic surface a resinous coating which increases in thickness or areal density as long as the surface is immersed in the composition. Various types of activators or activating systems are known, for example, as reported in the following U.S. Pat. Nos.: 3,592,699; 3,709,743; 4,103,049; 4,347,172; and 4,373,050, the disclosures of which, to the extent not inconsistent with any explicit statement herein, are hereby incorporated herein by reference. The activating system generally comprises an acidic oxidizing system, for example: hydrogen peroxide and HF; HNO.sub.3 ; a ferric-containing compound and HF; and other soluble metal-containing compounds, for example, silver fluoride, ferrous oxide, cupric sulfate, cobaltous nitrate, silver acetate, ferrous phosphate, chromium fluoride, cadmium fluoride, stannous fluoride, lead dioxide, and silver nitrate in an amount between about 0.025 and about 50 grams per liter ("g/l") and an acid, which can be used alone or in combination with hydrofluoric acid, and including, for example, sulfuric, hydrochloric, nitric, and phosphoric acid, and organic acids, including, for example, acetic, chloroacetic, and trichloroacetic acids.
Previously known autodepositing compositions can be used to form coatings which have good aesthetic properties and which protect the underlying metallic substrate from being degraded (for example, corroded by water). However, there are certain applications which require that the autodeposited coating have particularly good properties for satisfactory use. Various means have been developed to improve the properties of autodeposited coatings, including, for example: chemical pretreatment of the metallic surface prior to formation of the coating; selection of particular resins for use in forming the coating; addition to the autodepositing composition of chemical additives; and chemical treatment of the freshly formed or uncured coating, as described in detail in copending application Ser. No. 202,117 filed Jun. 3, 1988 and assigned to the same assignee as this application.
It is an object of this invention to provide metallic surfaces with autodeposited coatings with better adhesion and/or better corrosion resistance than those obtained by following the teachings of the prior art, particularly in environments where the autodeposited coating is subject to mechanical damage. In such situations, it has been observed that autodeposited coatings often are susceptible to spotwise failures of protection against corrosion.